1. Field of the Invention
The present invention pertains to optical fiber sensors for sensing fluid substances, such as water, and more particularly, it pertains to optical fiber sensors which utilize a swellable material in the presence of the fluid substance to be detected which swelling is arranged to mechanically cause a microbending in the optical fiber that can readily be detected and analyzed by conventional monitoring devices.
2. Description of the Prior Art
Optical fiber sensors for sensing the presence of a particular fluid substance such as water vapor, water, petrochemicals, etc., are well-known in the prior art. The presence of the fluid substance at a remote location, such as a buried telecommunications box, is sensed by a carefully chosen substance which is caused to thereby swell and mechanically create a stress in or, particularly, a microbending in the optical fiber which condition can then be readily detected by the attenuation in the signal transmitted by the fiber or by optical time domain reflectometer (OTDR) circuitry which reacts to the signal loss at the microbend and accurately pinpoints its location along the length of the optical fiber.
Prior United States patents which disclose the use of a fluid sensor employing the microbending of an optical fiber include the Pat. No. 5,243,670 to Bonicel wherein the presence of moisture is detected within a closed chamber; U.S. Pat. No. 4,596,443 to Diemeer et al disclosing a fiberoptic sensor for detecting the presence of a liquid such as ground water within a multiconductor cable; U.S. Pat. No. 5,378,889 to Lawrence disclosing a fiberoptic sensor for the detection of hydrocarbon fuels; and U.S. Pat. No. 5,430,815 to Shen et al which discloses an optical fiber sensor for detecting the presence of water wherein a swellable material forces the optical fiber to be bent about a curved mandrel. Optical fiber sensors utilizing the microbending of the fiber for detection purposes have also been used to detect shockwaves as shown, for example, in the prior patent to Sargoytchev, U.S. Pat. No. 5,220,160.
While in the foregoing patents the swellable material directly impacts upon the fiberoptic cable, the location for detecting the presence of particular fluid substance may be separated from the location of the fiber attenuation imparting means so as to prevent any contact therebetween with a mechanical connection being provided to impart a bending to the fiber upon the remote sensing of the fluid substance as shown, for example, in the U.S. Pat. No. 4,590,462 to Moorehead disclosing an optical fiber sensor for the detection of hydrocarbons.
A particular industry which has utilized optical fiber sensors in the past is the telecommunications industry wherein fiberoptic splice cases are used at remote locations as junctions or splice points for the fiberoptic cables. Typically, these splice cases are buried underground and are thereby subject to water intrusion, for example, as a result of physical damage, improper sealing or direct leakage through a cut in the cable sheath. Since such water instrusion can damage the optical fibers and the cladding thereof and can therefore have significant consequences on the accuracy and reliability of the information communicated over the fiberoptic cables, it is imperative that the presence of water in the splice case be communicated to some remote operating point as soon as possible.
A particular problem with prior art optical fiber sensors in dealing with the presence of water in splice cases is the lack of discrimination in the sensor in distinguishing between free water (which can produce the adverse consequences as pointed above) and water vapor (which will unavoidably occur and which is generally much less harmful). It is understandable that a high humidity can build up in objects like buried splice cases over time particularly during the wet seasons. If the cumulative effects of the continued presence of a highly humid atmosphere causes the sensor to be triggered even though no free water is present, unnecessary and costly replacements or repairs may occur.